Drilling Fluids Generally
Oil and gas wells are most commonly drilled using the rotary drilling method. In this method, a drill bit with fixed or rotatable cutting teeth is mounted at the lower end of a drill string, which is an assembly of drill pipe, drill collars, and other drilling accessories. The drill string is typically rotated by means of either a rotary table or a top drive apparatus associated with the drilling rig. In some cases, the drill string is rotated by what is commonly referred to as a mud motor. Whatever means of rotation is used, the rotation of the drill string causes the drill bit to bore into the ground. Additional sections of drill pipe are added to the drill string as the well is drilled deeper, until the desired depth is reached. The cutting diameter of the drill bit is larger than the diameter of the drill string components, so the drilling operation creates an annular space between the drill string and the earthen sides of the wellbore.
During rotary drilling operations, a slurry mixture called drilling fluid (commonly referred to as “drilling mud”) is circulated continuously down through the drill string, out the bottom of the drill string (through nozzles or jets near the cutting teeth of the drill bit) into the annular space between the drill string and the wellbore, and then back up to the surface. Drilling mud serves a number of important functions in the drilling operation. Of primary importance, the mud carries bored material (commonly called “cuttings”) out of the wellbore and up to the surface, so that the cuttings do not clog the wellbore and impede further drilling. In a typical drilling operation, the mud returning from the wellbore is processed through various types of cleaning equipment, such as shale shakers, centrifuges, desilters, desanders, degassers, settling chambers, and other apparatus well known in the well drilling industry. This process removes cuttings, formation gas, and other contaminants so that the cleaned and conditioned mud can be reused.
Drilling mud also lubricates and cools the drill bit, further facilitating efficient drilling operations. As well, drilling mud can serve the important function of preventing a blow-out if a well is drilled into a subsurface formation that contains high pressure. The weight of the column of mud in and around the drill string exerts hydrostatic pressure on the bottom of the well, proportionate to the density of the mud and the height of the mud column. If this pressure is great enough, it will counteract the formation pressure so that a blow-out cannot occur. The hydrostatic pressure exerted by the mud also helps to prevent unwanted materials from infiltrating the wellbore, a consideration which is particularly important when drilling through formations containing loose or easily fractured materials.
Another valuable function of drilling mud is formation protection. Properly formulated, drilling mud remains fluid as long as it is in constant circulation, but may form a gel or become more thixotropic when not being circulated. Because of these characteristics, as the mud is being circulated by the mud pumps, it will adhere to and solidify on the borehole walls, lining the hole with a thin protective cake that prevents or minimizes the risk of loose or disturbed formation materials sloughing into the well.
Oil-based drilling muds (or “oil muds”) may be necessary or beneficial in certain circumstances, such as when drilling through formations containing expansive clays that swell upon contact with water. However, water-based drilling muds (also called “water muds”) are used much more commonly. Besides water, the main ingredient of a typical water mud is a viscosity agent, usually a fine-grained clay, which mixes with the water to form a slurry. Bentonite, which consists predominantly of an expansive clay called montmorillonite, is widely used in water muds, although other types of clay may be used as well.
The clay also increases the density of the mud, thus enhancing its effectiveness for blow-out protection. Various other weighting materials, such as barite, hematite, or calcium carbonate, may also be added for this purpose.
Other substances which may be added to drilling muds, depending on the intended application and desired properties of the mud include drilling detergents, foaming agents, defoaming agents, and alkaline materials (for counteracting acidic contaminants which may enter the mud).
It is also common to add natural or synthetic polymer materials to water muds, for one or more purposes. The behaviour and effect of a polymer in a drilling mud generally depend on the size of the polymer's molecules and their charge (e.g., anionic, cationic, or non-ionic). Some polymers may have the beneficial effect of minimizing loss of fluid from the mud. Some polymers may decrease the viscosity of the mud, while others may act to cause flocculation of the clay in the mud, thus increasing viscosity. Some polymers may serve multiple functions. Water muds that have significant polymer content may also be referred to as polymer drilling fluids.
An oil, such as diesel oil or mineral oil, is commonly added to enhance a water mud's lubricating characteristics. As oils are insoluble (or “immiscible”) in water, they may be dispersed into the mud as emulsions. When an oil is emulsified into an aqueous carrier fluid (e.g., water, or water mud), the oil is broken up into many small particles or droplets which become uniformly dispersed, in suspension, throughout the fluid. Without emulsification, the particles or droplets would simply re-agglomerate due to attractive forces between the molecules, and the oil would separate from the water as a discrete liquid phase.
One or more chemical emulsifying agents (or “emulsifiers”) are commonly used to emulsify oils in drilling mud. Emulsifiers work by reducing the interfacial tension between the molecules of immiscible liquids. Some emulsifiers fall into the category of surface-active chemical agents called surfactants. Not all surfactants are emulsifiers, however. There are many different types of surfactants, and they may be added to drilling muds for various purposes, depending on the surfactants' particular characteristics. Surfactants are commonly classified according to their hydophile-lipophile balance (HLB) numbers. HLB numbers (which are determined on a scale of 1 to 40) provide a semi-empirical method of predicting the type of properties a surfactant will exhibit, depending on its molecular structure.
A hydrophilic molecule or material is one which has a surficial affinity for water. Clays, like bentonite, which are readily wetted by water, are hydrophilic materials. In contrast, a lipophilic molecule or material is one that has a surficial affinity for oils or oily substances. A surfactant that is effective to emulsify an oil in water will typically have a fairly high HLB number, whereas a surfactant effective in emulsifying water in oil will typically have a fairly low HLB number. Accordingly, the selection of surfactants to be used as additives in drilling muds will involve consideration of HLB numbers, depending on desired surfactant properties and effects.
Problems with Known Drilling Fluids
Water muds, in diverse known formulations, perform satisfactorily in many applications. However, a particular problem arises when drilling through oil-bearing sand formations, such as those which occur extensively in northern Alberta. These oil sand formations contain vast reserves of oil, but the oil is thick and heavy and therefore difficult to recover. Considerable success in heavy oil recovery from Alberta's oil sands has been achieved in recent decades by means of innovative methods of in-plant processing of oil sand excavated in bulk using open-pit mining techniques. However, the usefulness of such methods is limited to recovery from oil sand formations that are close enough to the surface for open-pit mining to be practical. Recovery from deeper oil sand formations requires an entirely different approach.
Conventional production well technology, which relies on crude oil flowing by gravity and/or pressure into production wells, does not work well or at all in bituminous oil sand formations. Being quite thick and heavy, the oil in these formations is typically too viscous, in its natural state, to flow out of the sand. If its viscosity is low enough to permit gravity flow, recovery rates tend to be very low. However, recovery of heavy oil from such formations can be significantly enhanced using a relatively new technology called steam-assisted gravity drainage (or “SAGD”, as it is commonly known in the industry).
SAGD is fairly simple in concept. Using well-known directional drilling methods, a horizontal production well is drilled through an oil sand formation. A steam-injection well with a perforated liner is drilled above and substantially parallel to the production well. Superheated steam is then injected into the oil sand formation (either at the heel and/or toe of the liner or through the perforations in the liner of the injection well), thereby heating the oil or bitumen in an affected region of the formation (or “steam chamber”) generally extending upward and outward from the injection well. This heating effect causes the oil or bitumen in the steam chamber to become less viscous, such that it will flow by gravity and/or pressure through the sand and into the production well through perforations in the production well liner, whereupon it can be pumped or raised to the surface using conventional methods.
Like other types of well-drilling operations, the drilling of SAGD wells entails the use of drilling mud. However, drilling in bituminous oil sand formations poses a number of practical problems that are not satisfactorily addressed by prior art drilling mud technology. The cuttings contain significant amounts of heavy oil or bitumen, which can clog the shale shaker screens and other mud-processing equipment. As a result, effective removal of cuttings from the mud is more difficult, and the ability to clean and reuse the mud is reduced or even precluded. This increases mud costs, because new mud must be added to the mud system to replace mud that cannot be effectively cleaned and must therefore be discarded. This gives rise to the further problem of disposal of the discarded mud, laden with substantial quantities of sand coated with heavy oil and bitumen. Disposal of this contaminated mud is considerably more difficult, from both practical and environmental standpoints, than disposing of the comparatively clean particulate material removed from the mud in more conventional drilling operations.
Furthermore, the cuttings are very sticky because of the thick oil and bitumen, and they tend to stick to the drilling pipe, well casing, and liners. The presence of these sticky cuttings in the mud increases drag forces on the drill string components, thereby increasing the power and torque required to rotate the drill string, increasing wear and tear on the rig's drive mechanism, and increasing rig service and maintenance requirements. The sticky cuttings and bitumen cause particular problems when running liners into a horizontal well, because they tend to build up in curved casing sections where the well changes direction from vertical to horizontal, often making it necessary to clear the build-up before it will be possible to run the liners into the horizontal section without difficulty.
These problems can be mitigated to some extent by circulating the bitumen-laden mud through a large mud cooler. This cools the bitumen in the mud to the point that it is no longer sticky enough to adhere to well components. The major drawback to this solution is expense, as the cost of operating a mud cooler can commonly be several thousand dollars per day.
The inventors are aware of one attempt to reduce the problem of bitumen-laden cuttings sticking to well components, by using a polymer drilling fluid containing approximately 0.3% by weight of a non-ionic surface-active agent called HME ENERGIZER® sold by Montello, Inc. of Tulsa, Okla. HME ENERGIZER® consists of about 10% to 30% surfactants and 70% to 90% hydrocarbon solvent, so the mud system treated with HME ENERGIZER® contained between 0.3 and 0.9 kilograms of surfactant per cubic meter of mud. However, this formulation did not prove effective. In such low concentrations, the HME ENERGIZER® could not emulsify oil and bitumen from the cuttings, and in higher concentrations it would make the mud too thick to be used in the field.
U.S. Pat. No. 5,634,984 (Van Slyke) discloses a cleaning fluid that incorporates surfactants having HLB numbers of at least about 8, and which is stated to be usable for cleaning oil-contaminated substrates, including oil-contaminated drill cuttings. The Van Slyke patent indicates that this fluid can also be incorporated into a drilling fluid, but it contemplates such use only in association with oil-based drilling fluids. The Van Slyke specification states that the oil-to-water ratio of drilling fluids incorporating the cleaning fluid can be as low as 50:50 by volume, while also stating that compositions of the cleaning fluid commonly contain less than 20 percent water and preferably less than 0.5 percent water. It is therefore readily apparent that while the Van Slyke compositions might be effective in certain applications, the Van Slyke patent does not address the problems caused by bitumen-laden cuttings in the specific context of water-based drilling fluids.
For the foregoing reasons, there is a need for a water-based drilling fluid that can be used for rotary drilling operations in oil sand formations, and which is capable of effectively removing oil- and bitumen-laden cuttings without the cuttings or the oil or bitumen contained therein adhering to drill string components and associated downhole equipment, in detrimental quantities or at all; without significantly increasing or decreasing the thickness or viscosity of the drilling fluid; with minimal or no increase in drag forces acting on the drill string; with minimal or no increase in the power needed to rotate the drill string; without reducing, significantly or at all, the suitability of the drilling fluid to be effectively cleaned using conventional mud-cleaning apparatus, and then reused in well-drilling operations; and without requiring the use of mud-cooling equipment to achieve these characteristics. The present invention is directed to these needs.